Microwave unilateral transducer



Oct. 11, 1960 J, DUNCAN MICROWAVE UNILATEIRAL TRANSDUCER 3 Sheets-Sheet1 Filed July 8, 1957 mvzmon BOBBY J. DUNCAN 3 Sheets-Sheet 2 Filed July8, 1957 mu H,

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MICROWAVE UNILATERAL. TRANSDUCER Filed July 8, 1957 3 Sheets-Sheet '3 1Mmgggm Maxi RNVENTOR BOBBY J. DUNCAN KMQQ/Ja ATTORNEY United StatesPatent C) MICROWAVE UNILATERAL TRANSDUCER Bobby J. Duncan, PortWashington, N.Y., assignor to Sperry Rand Corporation, a corporation ofDelaware Filed July 8, 1957, Ser. No. 670,586

3 Claims. (11. 333-24 This invention relates to unilateral transducersand more particularly to broad-band unilateral transducers for use inmicrowave transmission lines employing helical conductors.

A problem that has long existed in traveling-wave tubes is thatreflections of the propagating waves are created by mismatches in themicrowave circuit of the tube. These reflected waves, if of sufficientamplitude, will cause the tube to break into oscillation, thusdestroying its use as an amplifier. In order to reduce the amplitude ofthe reflected waves and to prevent the tube from oscillating, it hasbeen the practice in the past to introduce a lossy material into themicrowave circuit of the tube. This method has been successful toprevent the tube from oscillating. However, because the lossy materialis a bilateral attenuator, that is, it attenuates waves traveling in theforward direction as well as in the reverse direction, the gain of thetraveling-wave amplifier is reduced.

The development of unilateral attenuators employing ferrite materialsbiased to gyromagnetic resonance has led to the use of such devices intraveling-wave amplifiers to overcome the above-mentioned problem. Suchdevices are described in articles by J. S. Cook, R. Kompfner, and H.Suhl, entitled Nonreciprocal Loss in Traveling-Wave Tubes Using FerriteAttenuators, page 1188, Proceedings of the IRE, July 1954, and by J. A.Rich and S. E. Webber, entitled Ferrite Attenuators in Helices, page100, Proceedings of the IRE, January 1955. Such devices permit thepassage of the forward amplified waves in the traveling-Wave amplifierwith a negligible amount of attenuation, but greatly attenuate thereflected Waves, thus preventing oscillations from building up.

A traveling-wave amplifier is an extremely broad-band device; thisfeature often constituting an important factor in its use in a microwavesystem. However, known unilateral ferrite attenuators, or isolatorsemployed with traveling-wave amplifiers are not broad-band devices. Thishas the effect of limiting the useful bandwith of the traveling-waveamplifier to the bandwith of the ferrite isolator.

Therefore, it is an object of this invention to provide an improvedunilateral transducer.

Another object of this invention is to provide a broadband ferriteisolator for use in traveling-wave amplifiers.

Another object of this invention is to provide a broadband unilateralattenuator for use in microwave transmission lines employing a helicalconductor.

These and other objects which will become more apparent as thedescription proceeds are achieved by providing a ferrite element placedadjacent a helical transmission line in the region where the magneticfield of the microwave energy propagating on the transmission line iscircularly polarized. To achieve broadbanding eflects the ferriteelement may take the form of ahelix having a pitch which varies alongits length, or along a portion of its length. The ferrite helix isplaced about the R.F. transmission line. Alternatively, thehelix pitchmay be constant and the cross-sectional configuration of the ferritematerial forming the helix may vary.

In another embodiment of this invention the ferrite element may becomprised of a plurality of ferrite rings placed about the helicaltransmission line and located at spaced regions along the transmissionline. The ratio of the differential radius to the length of theindividual rings being different at the difierent spaced regions alongthe transmission line.

In still another embodiment of the invention the fer rite element maytake the form of a cylindrical tube disposed about the transmission lineand extending in a direction parallel to the longitudinal axis of thetransmission line; the differential radius of the ferrite tubebeing'different at spaced regions along its length.

A further embodiment of the ferrite element may be comprised of aplurality of regions extending parallel to the axis of the transmissionline; the ferrite material in each of said regions having a differentsaturation magnetization.

For a better understanding of the invention, reference is made to theaccompanying drawings wherein:

Fig. 1 is a diagrammatic representation of the R.F. magnetic field on ahelical transmission line;

Fig. .2 is a diagrammatic representation illustrating the internalmagnetization of ferrite elements having different shapes;

Fig. 3 is a series of diagrams of various ferrite configurations usefulin explaining the theory of operation of this invention;

Fig. 4 is a cross-sectional representation of one embodiment of thepresent invention; and

Figs. 5, 6, 7, and 8 are cross-sectional representations showing ferriteelements employed in alternative embodiments of this invention.

The theory of operation of microwave unilateral attenuators utilizingthe gyromagnetic resonance phenomenon in a ferrite element is now well.known in the art and may be briefly summarized as follows: if an R.F.magnetic field is circularly polarized in the positive sense in a planeperpendicular to the steady magnetization of a ferrite element, a largeabsorption of the microwave energy occurs at the value of themagnetizing field which brings the ferrite element into gyromagneticresonance.- This absorption of microwave energy does not occur when theR.F. magnetic field is circularly polarized in the negative sense.

Circular polarization is of the positive sense when the R.F. magneticfield rotates in a clockwise direction when viewed in the direction ofthe steady magnetization of the ferrite.

The above-cited articles point out that the R.F. magnetic field'on ahelical conductor is essentially circularly polarized in a thin regionon the outside of the helix and that by placing ferrite rings, or ahelical shaped ferrite element around the helical transmission line of atraveling-wave tube, and utilizing the magnetic focusing field of thetraveling-wave amplifier as the ferrite magnetic biasing field,unilateral attenuation can be achieved;

Byanalyzing the expressions for the components of the magnetic fieldaround a helical transmission line, as expressed on page 231 ofTraveling Wave Tubes, by I; R'. Pierce, published by D. Van NostrandCo., Inc., 19-50, J. A. Rich' and S. E. Webber have shown in theabove-cited article that the magnetic field may be considered asessentially circularly polarized in the r@ plane and in the rz plane,where r, 0, and z are the- The representation of the R.F. magnetic fieldaround alielical transmission line as shown in Fig. 1 will aid in;

demonstrating this fact Considering that propagation of the microwaveenergy is from left to right, the projection of the magnetic vector Aonto the rz plane will show circular polarization of the magnetic field.The circular polarization is of one sense, positive or negative, 'on theoutside of the helix, and of the opposite sense on the inside of thehelix. For propagation of micro wave energy from right to left, thesense of circular polarization, positive or negative, will be reversed.Also, the projection of the vector onto the rplane will show circularpolarization. In order to visualize this latter circular polarization itmust be realized that because of the pitch of the helix, the truerepresentation of the magnetic field is not parallel to the plane of thepaper but is at an angle with the plane of the paper.

As pointed out above, in order to achieve unilateral attenuationemploying the gyromagnetic resonance phenomenon the ferrite materialmust have a steady magnetization in a direction perpendicular to theplane of RF. circular polarization, and this steady magnetization mustbe suflicient to bring the ferrite into resonance atthe frequency ofoperation.

" The steady internal magnetic fields present in several ferriteelements having different physical configurations will next beconsidered in connection with Fig. 2. In

Fig. 2a is shown a ring of ferrite material positioned so' that theapplied steady magnetic field H is along the center axis of the ring. Inthis instance the ring of ferrite material will be magnetized in the zdirection only and there will be no 0 component, assuming that theferrite material is homogeneous and that the applied field is along thecenter axis of the ring. Similarly, in Fig. 2b, where a tube of ferritematerial is positioned so that the applied steady magnetic field isalong its center axis, the ferrite material will be magnetized in the zdirection only and will have no 0 component of internal magnetization. Aferrite element in the form of a helix is shown in Fig. 20. In thiscase, because of the high permeability of the ferrite material relativeto the air gap between turns of the helix, the internal magnetization ofthe ferrite will be along the pitch of the helix. The steady internalmagnetic field, H may be considered as made up of a 0 component and zcomponent. In -a helix with a moderate pitch angle 1, 3, the 0component'will be predominant, so that in the discussion to followhereinafter dealing with a ferrite helix having a moderate pitch angle,only 0 component of steady magnetic field need be considered.

'In considering the illustrations of Fig. 2, it can be seen that if thepitch angle 1,9 of the helix is appreciably increased, the z componentof the'internal magnetic field will increase since the pitch of thehelix will be more closely aligned in the z direction. Likewise, if thepitch angle 1/ is near zero, the 0 component will diminish and the 2component will be predominant. Thus by varying the pitch angle of thehelix, the steady internal magnetic field, hence the magnetization ofthe ferrite may be 'varied.

C. Kittel in an article On the Theory of Ferromagnetic ResonanceAbsorption, Physical Review, volume 73, No. 2, January 15, 1948, pages155-161, has shown that the gyromagnetic resonant frequency is dependenton the shape of the ferrite specimen. This dependency is related to thenature of the demagnetizing fields, which in depends on the shape of thespecimen. The general case' of magnetic resonant frequency is thus givenby [H,+(N,-Nz)Ms1 H..+( X Z) S where w =21rf and f is the gyromagnet-icresonant frequency, 'y is the magneto-mechanical ratio for an electronspin 7 l 2 21r oersted V m n, is the value of the steady magnetic fieldwhich is always directed the z direction for purposes, of

4 equation. The z direction in this equation corresponds to a directionperpendicular to the plane of the RP. magnetic field circularpolarization; N N N are the demagnetizing factors, the sum of whichequals 41r, and M is the saturation magnetization of the ferrite, anddepends on the ferrite composition.

Upon applying Equation 1 to various geometrical configurations offerrite materials, the change in gyromagnetic resonantfrequency may beillustrated. In Fig. 3, wherein H represents the applied steady magneticfield, two geometrical configurations are illustrated. In Fig. 3a, thesteady magnetic field is applied parallel to the axis of a long, thinrod whose diameter is small compared to -a wavelength in the ferritemedium. For this configuration and direction of applied field, N N =21r;

Nz=0, and

J w,;- [Ir,+2M,] (2) In Fig. 3b, the steady magnetic field is appliedtransyersely to the axis of the rod. For this situation N =N =21r; N =0,and

w [H (H,,-21|'M (3) In Fig. 3c, a thin ferrite slab is used, in whichthe slab thickness is small compared to a wavelength in the ferrite, andto the width and length of the ferrite samples. The magnetic field isapplied perpendicularly to a narrow edge of the slab. For this case, N=N =0;

In Fig. 3d, the static magnetic field is applied perpendicularly to theface of a thin ferrite slab. For this situation, N =N =0g N =41r, and

Thus it is seen that for a given ferrite material, speci menshavingdifferent geometrical configurations may have different gyromagneticfrequencies. These equa- :tions also show that different values ofsteady magnetic field and different values of saturation magnetizationresult in dilferent gyromagnetic resonant frequencies.

A device constructed in accordance with the present invention which willachieve broad-band unilateral attenuation is shown in diagrammatic formin Fig. 4 wherein the slow wave propagating structure of a travelingwavetube is shown as a helical transmission line 11 enclosed within a glassenvelope 12. Disposed about the helical transmission line and glassenvelope 12 and extending in a direction parallel to the longitudinalaxis of transmission line 11 is a ferrite element 13 having the form ofa helix. As is shown in Fig. 4, the cross-sectional configuration of theferrite material forming the helix varies from a rectangle 40 having itsmajor axis parallel to the longitudinal axis of the helical transmissionline to a rectangle 42 having its major axis perpendicular to thelongitudinal axis of the transmission line.

Enclosing the helical transmission line 11, glass envelope 12, andferrite helix 13 is an envelope 14 which may be either metal or glass;members 18 and 19 represent conventional electron gun and collectorelectrodes, respectively, of a traveling-wave tube.

Waveguide 15 provides an input for the RF. energy which couples tohelical transmission line 11. Waveguide 16 provides an output for theamplified R.F. energy. Focusing coil 17 surrounds the envelope 14 andprovides a longitudinal magnetic field for focusing the electron beam ofthe traveling-Wave tube, and also provides the applied steady magneticbiasing field H for the ferrite element 13.

It is to be understood that the cross-sectional configuration of theferrite material forming helix 13 may vary in such a manner that thecross-sectional area will take a form other than a rectangle. Forinstance, the crqss section may vary 511Gb that it is substantiallytriangular in shape overa portion of the length of the helix, or it maybe substantially circular over a portion of its length. Any of a numberof diverse cross-sectional configurations may be employed, and indeedmay have to be employed to obtain a desired bandwidth.

An alternative embodiment of a ferrite helix is shown in Fig. 5 wherethe ferrite material forming helix 13 has a substantially constantrectangular cross-section but the pitch of the helix varies along itslength.

Both of the above-described embodiments of a ferrite helix disposedabout the helical transmission line will provide broad-band attenuationof the reflected R.F. wave as will now be explained.

As was shown in connection with Fig. l, circular polarization of theR.F. magnetic field exists in the rz plane and in the r-0 plane. Also,the steady internal magnetic field which is established in the ferritehelix by the axial magnetic field has a 0 component and a 1 component.As stated above, in the case of a ferrite helix of constant, moderatepitch, such as shown in Fig. 4, only the 0 component of steady magneticfield need be considered. Therefore, the R.F. magnetic field circularpolarization in the rz plane and the 0 component of steady magneticfield will be considered for this explanation relating to a ferriteelement in the form of a helix.

Therefore, with these conditions present, and with the applied steadymagnetic field in the direction shown in Fig. 4 and with the propagationof microwave energy from left to right, the R.F. magnetic field will becircularly polarized in a negative sense on the outside of helix 11 inthe region where ferrite helix 13 is located. Under these conditions theforward amplified microwave will pass along the transmission linesubstantially unattenuated by the ferrite helix, but the reflected wavewill be greatly attenuated since the reflected wave propagates fromright to left and the R.F. magnetic on the outside of the helix 1-1 isthen circularly polarized ina positive sense.

Referring to Fig. 4, the cross-section of the material forming theferrite helix at cross-sectional area 40 is substantially a rectanglehaving its major axis parallel to the longitudinal axis of helix 11 andsimulates a flat slab immersed in a steady magnetic field directedperpendicularly to a narrow edge as in Fig. 3c. As explained above, thegyromagnetic resonant frequency is expressed as in Equation 4 by In thisexpression H, is employed rather than H,, since it is the 0 component ofthe steady magnetic field generated in the helix by the axial magneticfield which is used to bias the ferrite to the proper field to obtainunilateral attenuation.

At cross-sectional area 41 the material forming the ferrite helix can beconsidered as a thin, infinitely long rod immersed in a steady magneticfield parallel to the longitudinal axis of' the rod, as in Fig. 3a. Inthis case the gyromagnetic resonant frequency is expressed as inEquation 2 by At cross-sectional area 42 the material forming theferrite helix may again be considered as a thin slab immersed in asteady magnetic field directed perpendicularly to a narrow edge, whosegyromagnetic resonant frequency is again expressed by Equation 4.Because the ferrite element 13 is comprised of a plurality of regionshaving different cross-sectional configurations, and since theseconfigurations have difierent gyromagnetic resonant frequencies,broad-band unilateral attenuation will be achieved.

As shown in Fig. 5, the cross-sectional configuration .of the ferritehelix 13 is substantially square and is con- .stant throughout itslength, but the pitch angle 1/ of the helix varies along its length. Thehelix in this case simulates a long, thin rod magnetized in a directionparallel to its longitudinal axis, as shown in Fig. 3a. The gyromagneticresonance frequency for this configuration is expressed by Equation 2 byAs was pointed out in connection with Fig. 2, the steady internalmagnetization of the ferrite will vary as the pitch angle #1 is varied.Thus, the H, term in the above equation will vary as the pitch angle isvaried, and consequently the gyromagnetic resonant frequency of theferrite of Fig. 5 element will be different at different regions alongits length, and a broadbanding effect is achieved.

Another means for obtaining attenuation of the re flected wave over abroad band of frequencies is shown in Fig. 6. In this embodiment theferrite element is comprised of a plurality of ring-shaped members18-22, which are disposed around helical transmission 11 at spacedregions along its length. It may be seen that the ratio of thedifferential radius to the length of the individual ring-shaped membersvaries along the length of the transmission line. Differential radius isdefined as the dif ference in length between the inside radius and theoutside radius of a ring-shaped member.

In member 18 the differential radius is small compared to the length ofthe member, while in member 20 the differential radius is large comparedto its length. In member 19 the differential radius and length aresubstantially equal.

In practice it may be desirable to employ a ferrite element comprised ofmore or less ring-shaped members than are shown in Fig. 6; five membersare shown merely for purposes of illustration.

A broad-band unilateral transducer or isolator constructed in accordancewith this invention may take still another form, and is illustrated inFig. 7. In this embodiment, the ferrite element 13 disposed abouthelical transmission line 11 is in the form of a tube which extendsparallel to the axis of transmission line 11. It may be seen that aspaced regions along the length of the tube the differential radius ofthe tube is different.

As discussed in connection with Fig. 2, with the steady magnetic fieldapplied in the direction shown in that figure, the steady magnetizationof the ferrite ring and tube is in the z direction only. Therefore, inFigs. 6 and 7, circular polarization of the R.F. magnetic field in ther-0 plane, and the steady magnetization of. the ferrite element in the zdirection are the components which produce gyromagnetic resonance in theferrite element.

In Fig. 6 the individual ring can be considered approximately as eitherrods or sheets of ferrite material infinite in extent, depending uponthe ratio of the ring length to differential radius. In ferrite ring18'the length of the ring is large compared with its differential radiusand simulates a flat slab of ferrite material immersed in a steadymagnetic field directed perpendicularly to a narrow edge. For thisconfiguration the gyromagnetic resonant frequency is expressed inEquation 4 by In ring 19 the differential radius is substantially equalto the length of the ring and simulates a ferrite rod immersed in asteady magnetic field perpendicular to its longitudinal axis and thegyromagnetic resonant frequency is expressed as in Equation 3 by In ring20 the differential radius of the ringis large compared with the lengthof the ring and simulates a thin slab of ferrite material magnetizedperpendicularly to its broad face, and the gyromagnetic resonantfrequency is expressed by Equation 5 as 7 v Again it is evident thatabsorption of the microwave energy will occur over a broad band offrequencies as expressed by the above equations. The analogy made withthe rings of Fig. 6 may also be made with respect to the tube of ferriteshown in Fig. 7 by considering that th'e'tube is comprised of aplurality of regions spaced .along its length. These regions, whenconsidered individually, will be similar to the individual rings ofFig.. 6 and broad-band unilateral attenuation will also be achieved. .Itshould be noted that the ferrite tube of Fig. 7 may havethe elfect of.concentrating the applied magnetic field (the electron beam focusingfield) within the ferrite tube, thus resulting in a defocusing of theelectron beam of the traveling-wave tube. ,For this reason the ferriteelement is preferably made in the form of rings or a helix in which casethe above-noted difliculty is largely eliminated.

However, if a suitable ferrite material with a relatively low D.C.permeability is employed in the ferrite tube, the concentration of themagnetic field in the ferrite element may be reduced and objectionabledefocusing of the electron beam may be reduced to an acceptable level.

The above equation also indicate that different values of M will resultin different gyromagnetic resonant frequencies. Thus the physicalconfiguration of the ferrite element may be maintained constant and aplurality of ferrite materials having difierent saturationmagnetizations may be employed to obtain broad-band unilateralattenuation. For instance, in Fig. 4, the ferrite element may becomprised of a plurality of ferrite helices, each helix being made offerrite material which has a different saturation magnetization.Likewise, in Fig. 6, the individual rings of Fig. 6 may be made ofdifferent ferrite materials. Thus a broad-band unilateral transducer mayalso be constructed by employing a ferrite element which is'comprised ofa plurality of regions having different saturation magnetizations. It isknown that as the frequency of the microwave energy increases, the RF.wave is concentrated more closely to the helical transmission line. Thiscauses the region of maximum circular polarization of the R.F. wave tomove closer to the helical transmission line. To assure optimuminteraction of the microwave energy and the ferrite element at thehigher microwave frequencies, the ferrite element should be soconstructed that at least a portion of it is located in the region ofmaximum circular pol-arization at the higher frequencies. A deviceconstructed in accordance with this invention which will take this factinto account is illustrated in Fig. 8. The ferrite element is comprisedof a plurality of ring-shaped members wherein the inner surface of ring30 is closely adja cent the helical transmission line 11, and the innersurfaces of rings 31, 32, and 33 are progressively further away fromtransmission line 11. When the microwave frequency increases and theregion of maximum circular polarization moves closer to the transmissionline helix, there will always be a ferrite ring which is in position foroptimum interaction with the RR wave.

For optimum performance of this device ring 30 would have a value ofsaturation magnetization which would cause it to be gyromagneticallyresonant at a higher frequency, in which case the region of maximumcircular polarization is close to the helix 11. Similarly, rings 31, 32,and 33 would have values of saturation magnetization such that eachwould be gyromagnetically resonant 'at a progressively lower frequencysince the region of maximum circular polarization will move further fromhelix 11 as the frequency decreases.

1 Broadbanding of the unilateral attenuator, or isolator, may beachieved by employing a combination of any of the above-discussedmethods, and in some instances it may be necessary to rely on acombination of methods to achieve the desired bandwidth. For example,ferrite rings made of one ferrite material and a ferrite helix made of adifferent ferrite element may be employed.

The steady magnetization of the various ferrite elements described abovemay also be achieved by employing ferrite elements which are permanentlymagnetized, rather than utilizing the electron beam focusing field toprovide the steady magnetization. In such a case the ringshaped ferritemembers and the tubular ferrite element may be magnetized in the 6'direction or in the z direction. However, it is believed that it ispreferable to use the previously-discussed method in a traveling-wavetube be cause the magnetic fields associated with the permanentlymagnetized ferrite element are likely to have a detrimental effect onthe focusing of the electron beam in the tube.

While the invention has been described in its preferred embodiments, itis to be understood that the words which have been used are words ofdescription rather than 'of limitation and that changes within thepurview of the appended claims may be made without departing from thetrue scope and spirit of the invention in its broader aspects.

What is claimed is:

1. A unilateral transducer comprising a helical wire transmission lineadapted to propagate waves of microwave energy in a given band offrequencies, the magnetic field of said waves being substantiallycircularly polarized in a region surrounding said transmission line,ferrite material disposed coaxially about said wire helix in a pluralityof longitudinally spaced regions where the magnetic field of said Wavesis circularly polarized, the longitudinal dimensions of the ferriteprogressively increasing thnoughout substantially the entirelongitudinal extent of said spaced regions in a direction parallel tothe longitudinal axis of the wire helix and the radial dimensions of theferrite progressively decreasing throughout substantially the entirelongitudinal extent of said spaced regions in the same directionparallel to the longitudinal axis of said wire helix, and means forimmersing said ferrite in a steady magnetic field which is directedparallel to the longitudinal axis of said wire helix and of suflicientstrength to magnetize the ferrite in each of said spaced regions to adifferent gyromagnetic resonant frequency which is included in the givenband of frequencies.

2. The combination as claimed in claim 1 wherein said ferrite materialis in the form of a helix and the longitudinal and radial dimensions ofthe individual convolutions of the ferrite helix in a plane parallel tothe longitudinal axis vary in the manner described in claim 1. 3. Thecombination as claimed in claim 1 wherein said ferrite material is inthe form of a plurality of longitudinally spaced rings and the lengthsof the rings in the direction parallel to the longitudinal axis of thewire helix and the differential radii of the rings vary in the mannerdescribed in claim 1.

References Cited in the file of this patent 'ber 1956, pages 1368-1386.

Enander: Proceedings of the I.R.E., vol; 44, No. 10, October 1956, pages1421-1430.

Introduction to Solid State Physics (Kittel), published 7 by John Wileyand Sons (New York), 1956 (2nd ed),

(pages 158-161 and 410-414 relied on);

